The present invention relates generally to a composite electrode including a low heat loss and small contact area interface with a phase change media. More specifically, the present invention relates to a phase change media memory device in which a composite electrode includes an exposed portion in contact with the phase change media. The exposed portion comprises a small percentage of an overall area of the composite electrode such that there is a small area footprint between the exposed portion and the phase change media and the small area footprint reduces heat transfer from the phase change media to the composite electrode.
Memory storage devices based on a phase change material to store information are being considered as an alternative to conventional data storage devices such as hard discs and flash memory, just to name a few. In a phase change material based memory device, data is stored as one of two physical states of the phase change material.
For instance, in an amorphous state, the phase change material can represent a binary zero xe2x80x9c0xe2x80x9d and the state of the phase change material can be determined by passing a current through two electrodes in contact with the phase change material and sensing a voltage drop across the phase change material. If in the amorphous state, the phase change material has a high resistance, then the voltage drop will be high.
Conversely, the state of the phase change material can be altered to a crystalline state that represents a binary one xe2x80x9c1xe2x80x9d by passing a current of sufficient magnitude through the electrodes such that the phase change material undergoes Joule heating. The heating transforms the phase change material from the amorphous state to the crystalline state. As mentioned above, a voltage drop across the phase change material can be used to sense the state of the phase change material. Therefore, if in the crystalline state, the phase change material has a low resistance, then the voltage drop will be low.
Another way of expressing the state of the phase change material is that in the amorphous state, the phase change material has a low electrical conductivity and in the crystalline state, the phase change material has a high electrical conductivity.
Ideally, there should be a large enough difference between the high resistance of tne amorphous state and the low resistance of the crystalline state to allow for accurate sensing of the state of the phase change material. Moreover, in a memory device based on an array of phase change material storage cells, some of the storage cells will be in the amorphous state and others will be in the crystalline state. It is desirable to have a minimal variation in the high resistance among the storage cells in the amorphous state and to have a minimal variation in the low resistance among the storage cells in the crystalline state. If either variation is too large, it may be difficult or impossible to accurately sense the state of the phase change material.
In FIG. 1, a prior phase change storage cell 100 includes a first electrode 103, a second electrode 105, a dielectric 107, and a phase change material 101 positioned in the dielectric 107 and in electrical communication with the first and second electrodes (103, 105). Typically, the dielectric 107 forms a chamber that surround the phase change material 101. To alter the state of the phase change material 101 from an amorphous state a (denoted by vertical hash lines) to a crystalline state C (see horizontal hash lines in FIG. 2), a current I is passed through the first and second electrodes (103, 105). The flow of the current I through the phase change material 101 causes the phase change material 101 to heat up due to Joule heating J.
In FIG. 2, a heat H generated by the current I is primarily dissipated through the first and second electrodes (103, 105) because the first and second electrodes (103,105) are made from a material having a high thermal conductivity, such as an electrically conductive metal, for example. To a lesser extent, a heat hxe2x80x2 is dissipated through the dielectric 107 because the dielectric 107 has a lower thermal conductivity than the first and second electrodes (103, 105). For instance, the dielectric 107 can be a layer of silicon oxide (SiO2).
As the heat H flows through the phase change material 101, a portion of the phase change material 101 undergoes crystallization to a crystalline state C (denoted by horizontal hash lines), while another portion of the phase change material 101 remains in the amorphous state a.
One disadvantage of the prior phase change storage cell 100 is that not all of the energy contained in the Joule heat J is used in transforming the state of the phase change material 101 from the amorphous state a to the crystalline state C. Instead, a significant portion of the Joule heat J is wasted because it is thermally conducted away from the phase change material 101 by the first and second electrodes (103, 105). As a result, more current I is required to generate additional Joule heat J to overcome the heat loss through the first and second electrodes (103, 105).
Increasing the current I is undesirable for the following reasons. First, an increase in the current I results in increased power dissipation and it is desirable to reduce power dissipation in electronic circuits. Second, an increase in the current I requires larger driver circuits to supply the current I and larger circuits consume precious die area. In general, it is usually desirable to conserve die area so that more circuitry can be incorporated into an electronic circuit. Finally, in battery operated devices, an increase in the current I will result in a reduction in battery life. As portable electronic devices comprise an increasingly larger segment of consumer electronic sales, it is desirable to reduce current drain on battery powered electronics so that battery life can be extended.
In FIG. 3, a plurality of the prior phase change storage cell 100 are configured into an array to define a prior phase change memory device 111. Each storage cell 100 is positioned at an intersection of the first and second electrodes (103, 105), a plurality of which are arranged in rows for the second electrode 105 and columns for the first electrode 103.
In FIGS. 3 and 4, one disadvantage of the prior phase change memory device 111 is that during a write operation to a selected phase change storage cell denoted as 100xe2x80x2, a substantial portion of the heat H generated by the current I dissipates through the first and second electrodes (103, 105) and into adjacent phase change storage cells 100. Consequently, there is thermal cross-talk between adjacent storage cells 100. Thermal cross-talk can slow down a switching speed of the phase change memory device 111 and can cause the aforementioned variations in resistance among the storage cells 100.
Another disadvantage of the prior phase change memory device 111 is that a surface of the phase change material 101 has a large contact area CA with the first and second electrodes (103, 105) (only the second electrode 105 is shown) and that large contact area CA promotes heat transfer from the phase change material 101 into the first and second electrodes (103, 105).
In FIGS. 3 and 4, the contact area CA is the result of a large portion of a surface area of the phase change material 101 being in contact with the first and second electrodes (103, 105) such that the heat H transfers easily from the phase change material 101 into the electrodes. The large contact area CA also contributes to the aforementioned thermal cross-talk. Moreover, heat loss from any given storage cell 100, thermal cross-talk from adjacent storage cells 100, and the contact area CA acting individually or in combination can lead to wide variations in resistance among the storage cells 100. For instance, if one storage cell 100 has its phase change material 101 preheated due to thermal cross-talk and another storage cell 100 does not have its phase change material 101 preheated, then when the phase change material 101 of both cells undergoes Joule heating J, the preheated cell 100 will have a greater percentage of its phase change material 101 crystallized than the non-preheated cell 100. Consequently, there may be variations in resistance between preheated and non-preheated cells. As was mentioned previously, variations in resistance are undesirable.
Consequently, there exists a need for a conductor structure for a phase change media memory device that reduces transfer of Joule heat from the phase change media and that reduces the amount of current necessary to alter the state of the phase change media. There exists a need for a conductor structure that reduces variations in resistance among phase change memory cells in a array. There is also need for a conductor structure that reduces thermal cross-talk and that reduces the surface area of contact between a conductor and the phase change media.
The low heat loss and small contact area electrode structure of the present invention solves the aforementioned disadvantages and limitations. The disadvantages associated with heat loss due to heat transfer into the electrodes is solved by a composite electrode that includes an exposed portion that is in contact with a phase change media. The exposed portion is only a small percentage of an overall surface area of the composite electrode so that a contact footprint between the exposed portion and the phase change media is small relative to a surface area of the phase change media. Consequently, only a small area of the phase change media is in contact with the exposed portion of the composite electrode and heat transfer into the composite electrode due to Joule heating is reduced.
The disadvantages associated with increasing current to compensate for heat loss through the electrodes is also solved by the composite electrode of the present invention because the exposed portion thereof presents a low thermal conductivity path to heat generated in the phase change media.
Variations in resistance among cells of phase change media in an array are reduced by the composite electrode of the present invention due to a low thermal cross-talk resulting from minimal heat transfer to the composite electrode.
Additionally, the disadvantages associated with a large contact surface area between the prior phase change material and its electrodes are solved by the contact footprint between the exposed portion of the composite emitter and the phase change media of the present invention.